This invention is drawn to the field of mechanical energy absorbing devices, and more particularly, to a novel energy absorbing configuration that uniformly absorbs mechanical energy over a broad frequency band and a wide amplitude range.
Mechanical energy is released under a variety of circumstances including nuclear and conventional weapon detonation and collisions between two massive bodies such as marine vessels, among others. The energy propagates and, if unchecked, encounters property and personnel causing equipment damage and a threat to life and safety. A variety of mechanical energy impeding devices have been positioned between the region where the mechanical energy arises and the region where either the personnel or the equipment are located in an effort to keep the magnitude of the transmitted acceleration and load within acceptable bounds.
The stress-strain curves for honeycomb, a common energy absorbing device, have a large initial peak, called the compressive strength, at which buckling begins, and then oscillate about a fairly flat average. The oscillations are due to the successive formation and bottoming out of new pleats; the honeycomb deforms in an accordian-like fashion.
The frangible tube as an energy-absorbing device applies an axial load to one end of the tube while the other end is pressed over a die. The die is shaped so that the portion of the tube in contact with the die is split into segments and the segments are broken into small fragments. A fluctuating force is developed, but the average force is approximately constant.
The inverting tube as an energy absorbing device is an axially loaded tube having a flair on one end. The flaired end is rigidly clamped and the tube is turned inside-out by pushing the tube through the flaired end. The load-deflection curves are flat. In order to increase the energy absorbing capability of this type of device, "drag" elements around the outside of the tube may be added whereby the total energy loss can be made to be several times greater than that lost due to inverting only.
The linear spring elastic energy impeding device typically comprises a preselected length of helically turned wire which is enclosed in a cylindrical housing. Another type is a preselected length of a braided stainless steel wire rope closed upon itself.
Several problems are encountered in the use of the known mechanical energy impeding devices. The first problem is that many absorbers become stiffer as they are compressed and therefore transmit greater acceleration at higher shock levels. Another problem is that the elastic devices elastically absorb energy and protect equipment during the compression phase, but then they release the energy during the unloading phase which causes rebound problems.
A third problem with some devices is that they are frequency dependent. Certain frequencies of mechanical energy are passed due to the particular loading behavior of the device while others are effectively blocked. This obviously is undesirable for personnel and equipment which would tend to be jerked back and forth and which would be subject to large frequency-dependent peak accelerations. In addition, many of the devices are expensive and considerable alignment difficulties arise to assure that they deform in the prescribed manners. Thus it is desirable to provide an improved energy absorbing mount that is not subject to the disadvantages of the prior techniques.